Ethylene oligomerization can produce 1-hexene at high selectivity using homogeneous, single-site chromium catalyst systems, activated by a molar excess of alkyl aluminums such as methyl alumoxane (MAO) and modified methyl alumoxane (MMAO). The trimerization of ethylene to 1-hexene represents one method of manufacturing desired oligomer products. Similarly, 1-octene and other desired oligomer products can be produced in high selectivity via ethylene oligomerization using homogeneous chromium catalyst systems activated by an appropriate aluminum compound. Such selective oligomerization reactions have been performed for many years with numerous optimization efforts. Exemplary processes of the reaction chemistry include U.S. Pat. No. 7,157,612 and International Patent Publications WO 2007/092136 and WO 2009/060343.
A major challenge associated with the selective oligomerization of ethylene is control of the reaction to maximize production rates while maintaining selectivity to the desired oligomers and maximizing catalyst utilization rates. Many ethylene oligomerization processes produce unwanted byproduct polymer which can foul process equipment and cause other process problems. For example, in an unavoidable side reaction, a small fraction of the converted ethylene forms polyethylene. This polyethylene can take any or all of the following three forms: (1) coat surfaces of the reactor and associated piping; (2) flow out of the reactor in solution with the reaction mixture; or (3) flow out of the reactor as a suspended solid in the reaction mixture. In addition, the formation of this byproduct polymer can continue downstream of the reaction system due to the presence of the still-active homogeneous catalyst in the reactor effluent.
A number of procedures have been developed for dealing with the problems of byproduct polymer formation and the presence of active catalyst in the reactor effluent in ethylene trimerization processes. For example, U.S. Pat. No. 6,380,451 discloses a method for killing the catalyst after it leaves an ethylene trimerization reactor by contacting the reactor effluent with an alcohol. An excess of alcohol is required, with a 5:1 mole ratio of alcohol to total catalyst metals being preferred. The preferred alcohol is one with a high enough boiling point that it can be easily separated from the desired hexene product by distillation. This patent also discloses a method for cleaning the polymer and catalyst residues that deposit on the internal surfaces of the reactor. The polymer is removed by periodic washing with the reaction diluent at a temperature 60° C. to 70° C. higher than the trimerization reaction temperature. In addition, this patent discloses cyclohexane as the preferred diluent/solvent in the reactor, as a good solvent is preferred to keep the catalyst system in solution.
U.S. Pat. No. 7,157,612 discloses another method for recovering byproduct polymer. Precipitation of byproduct polymer within the reactor is minimized by operating the reactor temperature high enough to keep the polymer in solution, with a preferred temperature of at least 110° C. On leaving the reactor, the effluent is contacted with an alcohol to deactivate the catalyst system. The effluent can then either be cooled, in which case some of the byproduct polymer precipitates and can be separated by filtration, or kept hot so that polymer stays in solution. In either case, soluble byproduct polymer continues to the downstream distillation columns where it is distilled away from the reaction products, diluents, and alcohol. In this manner, the byproduct polymer ends up with the catalyst and heavy byproduct residues.
Some other trimerization catalyst systems have been developed which permit the ethylene trimerization reaction to be carried out with high selectivity. For example, U.S. Pat. Nos. 8,067,609 and 8,138,348; and U.S. Patent Application Publications 2008/0058486, 2008/0188633, and 2008/0200743 all disclose the use of catalyst systems which are soluble in light paraffins, such as C3 to C6 iso- and normal-paraffins, and which exhibit high activities and improved selectivities at very moderate temperatures of 60° C. to 80° C. The use of such light solvents and mild reaction temperatures results in at least a portion, if not most or all, of the byproduct polymer being formed as an insoluble precipitate. Some of the insoluble polymer precipitates on the surfaces within the reactor and in the outlet piping. The insoluble polymer which does not stick to these surfaces exits the reactor as a suspended solid.
Prior art methods for dealing with the presence of active catalyst and byproduct polymer in the trimerization reactor effluent have problems. For example, the Cr-based trimerization catalysts typically employ an excess of aluminum alkyl activator relative to Cr compounds. In some systems, this excess can be 100:1 molar equivalents of Al to Cr or more, even up to 1000:1. The alcohols used to deactivate the catalyst are not selective to the Cr compounds since the alcohol also reacts with the Al compounds. An excess of alcohol over both Cr and Al is therefore required to ensure all active Cr species have reacted. The '451 and '612 patents discussed above, for example, teach use of a 5:1 molar excess of alcohol to total metals. If the Al:Cr ratio is 200:1 and a molar excess of alcohol to total metals of 5:1 is employed, then the molar excess of alcohol to Cr compound is 1000:1. This exceedingly high excess requirement for alcohol is costly, and also requires the addition of a distillation column for recovery of the unreacted alcohol for efficient utilization of the alcohol.
Prior art methods for separating byproduct polymer from the reactor effluent can also be cumbersome. Not all of the polymer contained in the reactor effluent can be separated by filtration, even if the effluent is cooled. Some of the polymer is still in solution, which carries through to the distillation columns. As the polymer is concentrated through successive distillation steps to recover reactants, products, and diluents, the polymer can precipitate and foul the column internals and reboilers.
Finally, build-up of byproduct polymer remaining within the oligomerization reactor and associated reactor piping can be troublesome. After byproduct polymers like polyethylene have fouled internal reactor surfaces and piping, it may become necessary to shut down the reactor(s) and wash the reactor(s) and piping out with a suitable solvent or wash liquid which can remove built up byproducts. Shutting down the reactor(s) for cleaning and maintenance is, of course, economically undesirable.
The byproduct polymer can be washed from the process using a hot solvent. In one embodiment of an ethylene trimerization process, as disclosed in WO 2011/112184, which is incorporated in full herein by reference, the byproduct polymer ends up as a dilute mixture with this solvent. A method to separate the byproduct polymer from this mixture and thus recover the solvent for recycle to the process, without fouling process equipment, is needed. This disclosure is directed to such a method.